Alkaline electrochemical cells typically contain an alkaline electrolyte such as potassium hydroxide (KOH), a cathode comprised of a metal oxide such as manganese dioxide, (MnO.sub.2), and a zinc anode. These cells provide excellent performance and are used throughout the world in many consumer applications.
A detrimental characteristic of these cells is the formation of hydrogen gas. This gas can be formed by undesirable corrosion of the zinc. As the quantity of gas increases, the internal pressure in the cell also increases. If this pressure increase is not relieved, the cells can eventually leak.
A widely used solution to the corrosion of the zinc is the addition of mercury to the anode as a corrosion inhibitor. Mercury effectively inhibits the corrosion of zinc and the formation of gas, and thus enhances the performance of the electrochemical cell. Unfortunately, as is well known, mercury may add to environmental pollution.
To address this environmental concern, the amount of mercury used in the cell has been lowered by adding zinc corrosion inhibitors to the cell. Examples of these inhibitors include lead, indium, cadmium, thallium, gold, silver, tin, gallium and compounds that incorporate these elements. These inhibitors have been alloyed with the zinc, deposited on the zinc, included in the electrolyte and deposited on the collector. Organic inhibitors, such as polyethylene glycol, have also been tried. These methods have led to the commercialization of low mercury content batteries, called "Ultra-low Mercury," that have about 250 parts of mercury per million parts based on total battery weight.
Unfortunately, the inhibitor methods employed do not reliably permit the total removal of mercury from the cell. For example, indium is an effective inhibitor at certain levels of mercury, but surprisingly, at lower levels of mercury, indium, is not as effective. Also, indium does not function well as a inhibitor for undischarged cells. The organic inhibitors are effective for cells that are undischarged and stored, but they do not inhibit gassing for cells that are partially discharged and then stored.
In view of these disadvantages, an alkaline electrochemical cell that is substantially free of mercury has not been commercialized. Moreover, considering the environmental concerns, such a cell is still desired.